Adhesive compositions comprising electrically insulating-coated carbon-based particles and methods for their use and preparation

ABSTRACT

Adhesive compositions that contain thermally conductive carbon-based materials that are also electrically insulated; methods for using such adhesive compositions and methods for their preparation.

BACKGROUND

The field of this disclosure relates generally to adhesive compositionsand, more particularly, to adhesive compositions that contain thermallyconductive carbon-based materials that are also electrically insulated.Other aspects of the disclosure relate to methods for using suchadhesive compositions and methods for their preparation.

Manufacture of electronic devices conventionally requires use ofadhesive compositions. The adhesive compositions are used to attachelectronic components (e.g., chips and resistors) to a target surfacewithin the device such as a mounting surface or heat sink surface. It isdesirable, in some instances, to use a thermally conductive adhesivecomposition (i.e., a composition that transmits heat well) so that theheat generated by the electronic component may easily pass through theadhesive to a heat sink (e.g., an aluminum or copper alloy in thedevice) and overheating of the device may be prevented.

Thermally conductive materials are typically also electricallyconductive. As a result, when a portion of the adhesive (andparticularly the filler material of the adhesive such as various metals)in conventional electronic devices dislodges due to age or repeatedtransport of the device (e.g., as in handheld devices or devices used intransportation), the electrically conductive components of the adhesivemay contact an active region of the device and may cause the device toshort-circuit. Such events may potentially cause failure of the device.Furthermore, if the composition of the adhesive is adjusted to increaseits thermal conductivity (e.g., by incorporation of greater amounts ofmetal), the electrical conductivity of the adhesive may increase to apoint at which the adhesive becomes electrically conductive causing ashort-circuit of the device. Accordingly, the material is often packagedin ceramic or plastic packaging which increases the cost of the device.

A need exists for adhesive compositions that are not electricallyconductive but which have a high thermal conductivity, allowing fortransfer of heat from a heat generating component to a heat sink. A needalso exists for processes for preparing such adhesive compositions andfor using the compositions in electrical devices.

BRIEF SUMMARY

One aspect of the present disclosure is directed to an adhesivecomposition suitable for use as a thermal conductor for electroniccomponents. The composition includes carbon-based particles at leastpartially coated with an electrically-insulating coating.

Another aspect of the present disclosure is directed to a process forproducing a thermally conductive adhesive composition. The processincludes depositing an electrically-insulating coating on carbon-basedparticles and mixing the coated carbon-based particles with a binder.

In a further aspect, a process for producing an electronic deviceincludes applying an adhesive composition to a target surface. Theadhesive composition includes a binder and carbon-based particles atleast partially coated with an electrically-insulating coating dispersedthroughout the binder. A component is applied to the adhesivecomposition to adhere the component to the target surface. The adhesivecomposition is then cured.

The features, functions, and advantages that have been discussed can beachieved independently in various embodiments of the present disclosureor may be combined in yet other embodiments further details of which canbe seen with reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of a carbon fiber with a boron nitride coatingthereon and prepared according to Example 1;

FIG. 2 is a photograph of the end of the carbon fiber of FIG. 1;

FIG. 3 is a graphical depiction of the EDAX analysis of a coated carbonfiber prepared according to Example 1; and

FIG. 4 is a photograph of a carbon fiber with boron nitride crystalsthereon and prepared according to Example 3.

DETAILED DESCRIPTION

Provisions of the present disclosure include adhesive compositionssuitable for use in electronic components and methods for preparing andusing such compositions. Generally, the composition includescarbon-based particles that are coated with an electrically-insulatingcoating. Such a coating protects devices in which the particles are usedfrom causing a possible short-circuit due to the electrical conductivityof the particles or from a portion of the adhesive (e.g., individualcoated particles or groups of such particles) dislodging. Thecarbon-based particles may be dispersed within one or more bindermaterials that imparts the adhesive quality to the composition.Carbon-based materials are used due to their relatively high thermalconductivity, light weight (i.e., low density) and resistance tooxidation and melting as compared to metal materials.

As referred to herein, “carbon-based particles” refers to particles thatare composed of at least about 50% by weight carbon. In otherembodiments, the carbon-based particles comprise at least about 75%, atleast about 90% or even at least about 95% by weight carbon. Thecarbon-based particles may have a thermal conductivity of at least 500W/m*K and, in other embodiments, have a thermal conductivity of at leastabout 1000 W/m*K, at least about 2000 W/m*K, at least about 4500 W/m*Kor even at least about 6000 W/m*K. The electrical resistance of thecarbon particles may be less than about 100 ohms*cm, less than about 50ohms*cm, less than about 30 ohms*cm, less than about 1 ohms*cm or evenless than about 0.1 ohms*cm.

The source of carbon-based particles used in the adhesive may be avariety of materials including, for example, carbon fibers, graphite,exfoliated graphite, graphene and combinations thereof. In certainembodiments, the particles are carbon fibers such as, for example,ThermalGraph® DKD fibers available from Cytec Industries Inc. (Stamford,Conn.). The carbon-based particles may take a variety of shapesincluding platelets, fibers, spheres, flakes, tubes, rods and diamonds;however, any shape may be used without departing from the scope of thepresent disclosure. Generally, anisotropic materials such as, forexample, fibers, rods and platelets are preferred as these materialsprovide high thermal conductivity at low incorporation rates (i.e., at alower relative volume of material). Electrically-insulating carbon-basedmaterials may be used; however, these materials are less preferred asthey typically are less thermally conductive. In certain embodiments,the carbon-based material is graphitic carbon and, in other embodiments,diamonds (natural or synthetic). In some embodiments, carbon nanotubesare used in the adhesive composition.

The particle size of the carbon-based particles may vary upon thedesired properties of the adhesive (adhesive power, thermalconductivity, viscosity, etc.) and may generally be determined by one ofordinary skill in the art. In some embodiments, the largest dimension ofthe particles is less than about 1 mm on average and may be less thanabout 500 μm, less than about 250 μm, less than about 100 μm or evenless than about 1 μm, on average. In embodiments where fiber particlesare used, the fibers may be less than about 100 nm (e.g., less thanabout 50 nm or from about 5 nm to 20 nm) in diameter and may be lessthan about 500 μm (e.g., less than about 300 μm or from about 200 μm toabout 300 μm) in length on average. When carbon rods are utilized, therods may be less than about 100 nm (e.g., less than about 50 nm or fromabout 5 nm to about 50 nm) in diameter and may be less than about 5 μmin length (e.g., less than about 3 μm or from about 500 nm to about 2μm) on average. When platelets are used, the largest width of the platesmay be less than about 100 μm (e.g., less than about 50 μm or from about10 μm to about 40 μm) and the thickness of the plates may be less thanabout 50 nm (e.g., less than about 20 nm or from about 5 nm to about 40nm). Generally, particle sizes other than those listed above may be usedwithout limitation in accordance with the present disclosure.

The carbon-based particles may be coated with an electrically-insulatingcoating. The coating may cover a portion of each particle or the entiresurface of the particle and may cover some of the particles or all ofthe particles without limitation. For purposes of the presentdisclosure, an “electrically-insulating coating” is a coating thatreduces the electrical conductivity of the carbon-based particles toless than that of uncoated carbon-based particles and, stateddifferently, increases the electrical resistance to more than that ofthe uncoated carbon-based particles. Electrical resistance may bemeasured, for example, by the testing method described in Example 2below. The electrical resistance of the carbon particles after coatingmay be at least about 50 ohms*cm and, in other embodiments, is at leastabout 100 ohms*cm, at least about 1000 ohms*cm, at least about 1×10⁵ohms*cm or even at least about 1×10⁸ ohms*cm. The thermal conductivityof the coated particles may be at least about 50 W/m*K, at least about300 W/m*K, at least 500 W/m*K, at least about 2000 W/m*K, at least about4500 W/m*K or even at least about 6000 W/m*K.

Suitable coating materials include, for example, ceramics such as metaloxides, metal nitrides, metal carbides and combinations thereof. Theceramic material may be, for example, boron carbide, boron nitride,silicon carbide and silicon nitride and, in some embodiments, is boronnitride. Generally, coating materials should be chosen which do notdegrade the thermal conductivity of the carbon-based particles, i.e.,the coating material should not be thermally insulating.

The electrically-insulating coating may be deposited on the carbon-basedparticles by any known method for depositing a coating and, in certainembodiments, is deposited by chemical vapor deposition. Chemical vapordeposition (CVD) is generally known by the skilled artisan and a numberof CVD techniques may be utilized such as, for example, plasma-enhancedCVD (PECVD), atmospheric-pressure CVD (APCVD), low- or reduced-pressureCVD (LPCVD), ultra-high-vacuum CVD (UHVCVD), atomic layer deposition(ALD) and aerosol assisted CVD (AACVD). The composition may be appliedby techniques other than CVD including, for example, sputtering. Liquidphase methods including sol-gel processes may also be utilized withoutdeparting from the scope of the present disclosure.

Generally, CVD methods involve introduction (typically continually) ofprecursor compounds at elevated temperatures at or near the surface ofthe substrate to which the coating is to be applied. The precursorcompounds react to deposit the coating on the substrate surface. Thecoating grows in thickness until compounds are no longer passed over thesubstrate at elevated temperatures. The precursor compounds chosen foruse depend on the desired composition of the coating and may generallybe determined by one of ordinary skill in the art. For instance, when aboron nitride coating is desired, compounds containing both boron andnitrogen may be used such as, for example, borazine (B₃N₃H₆).Alternatively, a first compound containing boron and a second compoundcontaining nitrogen may be contacted with the carbon-based particles.Boron compounds include, for example, boron trichloride (BCl₃), diborane(B₂H₆) and all compounds of the generic formula B_(x)H_(y). In certainembodiments, x is from 1 to 10 and y is from 1 to 15. Examples ofcompounds of formula B_(x)H_(y) include BH₃, B₂H₄, B₂H₆, B₃H₈, B₄H₁₀,B₅H₉, B₅H₁₁, B₆H₁₀, B₆H₁₂, B₈H₁₂, B₉H₁₅ and B₁₀H₁₄. Nitrogen compoundsinclude, for example, N₂, NH₃ and hydrazine (N₂H₄).

Similarly when silicon nitride coatings are desired, a boron compoundand a nitrogen compound may be used as precursor compounds. Suitablesilicon compounds include, for example, silicon tetrachloride, silaneand halosilanes (e.g., trichlorosilane).

The entire surface (or the entire surface exposed to the precursorcompounds) of the carbon-based particles may be coated or alternativelyonly a portion may be coated. Generally, it is desirable to coat as muchof the particles as possible to ensure that the particles containsufficient electrical-insulation. The thickness of the coating may bevaried to ensure adequate electrical resistance of the adhesivecomposition and to ensure thermal conductivity is not degraded belowdesired levels. Generally, the electrically-insulating coating may be atleast about 10 nm in thickness and, in other embodiments, may be atleast about 100 nm, at least about 500 nm, at least about 1 μm, at leastabout 10 μm or at least about 100 μm. In some embodiments, the thicknessof the coating is from about 10 nm to about 10 μm or from about 100 nmto about 1 μm.

The electrically-insulating coating may be applied at a variety oftemperatures and pressures depending on the deposition technique, thematerial to be deposited, the carbon-based particles used as thesubstrate, the desired coating thickness and the like. In variousembodiments, the coating is applied at a temperature of from about 450°C. to about 1000° C., preferably from about 500° C. to about 900° C. oreven more preferably from about 600° C. to about 800° C.

In some embodiments, the coating is applied at reduced pressures suchas, for example, less than about atmospheric, less than about 10⁴ Pa,less than about 250 Pa, less than about 1 Pa, less than about 10⁻³ Pa oreven less than about 10⁻⁶ Pa. The coating may be applied at atmosphericpressures or even at pressure above atmospheric such as at least about2×10⁵ Pa.

The adhesive composition may include a binder. Generally, the binderacts to add structural integrity to the adhesive and provides theadhesive force to attach surfaces and/or components. Generally, anyadhesive material may be used and, in certain embodiments, the binder isselected from epoxies, polyurethanes, acrylics and combinations thereof.One suitable adhesive is DIS-A-PASTE 2310 available from AptekLaboratories, Inc. (Valencia, Calif.).

The adhesive composition may be prepared by adding theelectrically-insulated coated fiber-based particles to the binder andmixing. In certain embodiments, high-shear mixing should be used tothoroughly mix the composition. One or more additives may be added andmixed before or after addition of the fiber-based particles. Suitableadditives include wetting compounds, surfactants, antifungal compounds,UV protectant compounds, defoaming compounds and catalysts. Generally,the adhesive composition is in the form of a paste or highly viscousliquid after mixing and prior to curing.

The adhesive composition may include at least about 10% coatedcarbon-based particles by weight with the remainder of the compositionbeing binder or additives. In other embodiments, the compositioncomprises at least about 25%, at least about 50%, at least about 75% oreven at least about 95% coated carbon-based particles. Generally, thepercent inclusion of components of the adhesive composition describedherein are listed as the percent inclusion by weight of the totalcomposition unless stated otherwise.

The coated carbon-based particles may be mixed into the binder withuncoated particles to minimize cost. Use of uncoated particles mayelevate electrical conductivity with the rise in conductivity beingproportional to the amount of incorporation of uncoated particles.Generally, however, the uncoated carbon-based particles should not beincluded in an amount above that which corresponds to a targeted ordesired threshold electrical conductivity. The adhesive composition maycomprise at least about 25% carbon-based particles coated according tothe present disclosure and, in other embodiments, comprises at leastabout 50%, at least about 75%, at least about 90%, at least about 95%,or even at least about 99% carbon particles with anelectrically-insulated coating by weight. In some embodiments, onlycoated carbon-based particles are used in the adhesive composition.

Once the adhesive composition is prepared, it may be applied to a targetsurface and a component or other surface may be contacted with theadhesive. Upon curing, the component is attached to the surface.Generally, curing times may be at least about 5 seconds, at least about30 seconds, at least about 1 minute, at least about 5 minutes, at leastabout 30 minutes, at least about 1 hour, at least about 5 hours or evenlonger depending on the binder.

In certain embodiments, a second binder material is added to the mixtureto cause the adhesive composition to begin to cure. For example, thecoated carbon-based particles may be mixed with a first binder toproduce a paste composition and immediately prior to use a second bindermay be added to the mixture to cause the binders to react and for themixture to begin to cure.

The adhesive composition may be used in electronic devices and mayreplace conventionally used adhesives. The adhesive composition may beused to attach an electronic component (e.g., integrated circuits,chips, resistors, thermal chip strap and the like) to a substrate (e.g.,dielectric layer) or to attach various layers in the device (e.g., as inmulti-layer PCB's. The adhesive may be used in applications where a heattransfer pathway is desirable such as between an electronic componentand a heat sink for that component. The adhesive composition may be usedin electronic devices used in the airline or satellite industries due totheir characteristic light weight, high durability and short-circuitprotection. While the adhesive compositions of the present disclosurehave been generally described in use with electrical devices, other usesare contemplated and within the scope of the present disclosure.

EXAMPLES Example 1 Preparation of Electrically-Insulated Carbon Fibers

A quartz tube was filled with carbon fibers (ThermalGraph® DKD, CytecIndustries Inc. (Stamford, Conn.)) (0.50 g) and placed in an aluminumoxide boat. The tube and boat were placed in a tube furnace. An argongas line was connected to one end of the quartz tube. The argon line wasconnected to a borazine bubbler (and bubbler by-pass) to add borazine tothe gas stream prior to addition to the quartz tube. The other end ofthe quartz tube exhausted into an oil bubbler and a fume hood.

The quartz tube was evacuated and backfilled with argon gas three times.Argon (100 ml/min) was flowed through the tube through the carbon fiber.The furnace temperature was raised to 700° C. and argon gas bubbledthrough the borazine bubbler (100 ml/min) was fed through the tube for15 minutes. After the minutes, the bubbler was bypassed and the tubefurnace was allowed to cool to room temperature. As can be seen fromFIG. 1, the carbon fibers were coated with boron nitride. The end of thecarbon fiber is shown in FIG. 2. The boron nitride coating is shown bethe darker cells (i.e., crystallites of boron nitride) to the right ofthe Figure. The coating forms a continuous layer on the carbon fiber.The boron nitride either displaced the surface carbon atoms or coatedthe surface carbon atoms. The coated carbon fibers were analyzed byenergy dispersive spectroscopy (EDAX) and the results are shown in FIG.3. As can be seen from FIG. 3, the coated fiber contained boron,nitrogen and carbon which is further evidence that the fiber was coatedwith boron nitride. Borazine is known to decompose into boron nitride.The small amount of oxygen observed may have been introduced from thealuminum tube or from a small amount of material being oxidized.

Example 2 Determination of the Electrical-Insulating Quality of theCoated Carbon Fibers

The coated carbon fibers of Example 1 (20 mg) were pressed into 7 mmpellets in a FTIR pellet press. Uncoated carbon fibers (20 mg) were alsopressed into 7 mm pellets. The pellets were mounted on transparentplastic tape and the conductivity of each pellet was tested using afour-point probe. Untreated carbon pellets had an electrical resistanceof 285 ohm and the coated pellets of Example 1 had a substantiallyhigher electrical resistance of 1.75×10⁶ ohm.

Example 3 Determination of the Effect of Increasing the CoatingDeposition Temperature

Example 1 was repeated but the furnace was heated to 1000° C. ratherthan 700° C. As can be seen from FIG. 4, crystals formed on the surfaceof the fibers. The crystals formed a non-uniform coating on the fibers.EDAX analysis confirmed that the crystals were composed of boron andnitrogen and that exposed portions of the fiber was carbon.

Example 4 Preparation and Electrical Resistance Testing of an AdhesiveContaining 5% by Weight Electrically-Insulated Carbon Fibers

The electrically-insulated carbon fibers of Example 1 were made into athermally conductive adhesive with an epoxy base. Thirty-minute DelayedSet Epoxy (part number SY-SS; Super Glue Corporation (Rancho Cucamonga,Calif.)) was used as the base. The epoxy is a two part formulationcontaining a resin and a hardener. The resin (0.982 g) and the coatedcarbon fibers (0.103 g) of Example 1 were mixed together by hand with aspatula. Hardener (0.984 g) was mixed into the resin and carbon fibersuspension with a spatula until a smooth black liquid was obtained. Thismixture resulted in a 5% by weight filler adhesive formulation which istypical for carbon filled thermally conductive adhesives. The filledadhesive was applied on a glass slide as a film using a 0.040 inch (1mm) gap draw down bar and was allowed to cure for 48 hours. The curedfilm was black, appeared uniform, and was 0.034 inches (0.86 mm) high.The electrical resistance was measured with a four point probe stationand found to be equivalent to an open circuit (the maximum resistancethe meter can measure is 1 G ohms, thus the electrical resistance of theepoxy is at least 10⁹ ohm*cm) which demonstrates that the adhesive waselectrically-insulating.

Example 5 Preparation and Electrical Resistance Testing of an AdhesiveContaining 10% by Weight Electrically-Insulated Carbon Fibers

An adhesive was prepared and tested according to the method of Example 4with different amounts of resin (0.271 g), coated carbon fibers (0.060g) and hardener (0.276 g). The mixture resulted in a 10% by weightfiller adhesive formulation which is also typical for carbon filledthermally conductive adhesives. The cured film was black, appeareduniform, and was 0.031 inches (0.79 mm) high. The electrical resistancewas also found to be an open circuit with the electrical resistance ofthe adhesive being at least 10⁹ ohm*cm.

Example 6 Preparation and Electrical Resistance Testing of an AdhesiveContaining 10% by Weight Non-Insulated Carbon Fibers

Untreated carbon fibers (ThermalGraph® DKD, Cytec Industries Inc.(Stamford, Conn.)) were incorporated into an adhesive and testedaccording to the method of Example 4. The adhesive contained resin(0.620 g), uncoated carbon fibers (0.137 g) and hardener (0.616 g) tomake an adhesive containing 10% by weight uncoated carbon fibers. Thecured film was black, appeared uniform, and was 0.025 inches (0.64 mm)high. The electrical resistance was measured to be 17×10⁶ ohms*cm whichis significantly lower than the adhesive containing coated carbonfibers.

Example 7 Preparation of Electrically-Insulated Carbon Fibers in thePresence of a Plasma

A quartz tube was filled with carbon fibers (ThermalGraph® DKD, CytecIndustries Inc. (Stamford, Conn.)) (0.50 g) and placed in an aluminumoxide boat. The tube and boat were placed in a tube furnace. An argongas line in a fitting with an electrical connection was connected to oneend of the quartz tube. Two steel wires extended from the electricalbypass into the tube furnace inside a ceramic jacket. The tips of thewire were exposed at the end of the ceramic jacket and used to strike aplasma. The wires were connected to a 1500V, 0.5 A power supply. Theargon line was connected to a borazine bubbler (and bubbler by-pass) toadd borazine to the gas stream prior to addition to the quartz tube. Theother end of the quartz tube exhausted into an oil bubbler and a fumehood.

The quartz tube was evacuated and backfilled with argon gas three times.Argon (100 ml/min) was flowed through the tube through the carbon fiber.The furnace temperature was raised to 700° C. and 1500 V was applied tothe wires until a plasma was generated. Then argon gas was bubbledthrough the borazine bubbler (100 ml/min) and was fed through the tubefor 15 minutes. After the minutes, the bubbler was bypassed and the tubefurnace was allowed to cool to room temperature. The carbon fibers cameout of the furnace with a white-yellow coating that was identified asboron nitride using EDAX in a scanning electron microscope (SEM).

This written description uses examples to disclose various embodiments,which include the best mode, to enable any person skilled in the art topractice those embodiments, including making and using any compositions,devices or systems and performing any incorporated methods. Thepatentable scope is defined by the claims, and may include otherexamples that occur to those skilled in the art. Such other examples areintended to be within the scope of the claims if they have structuralelements that do not differ from the literal language of the claims, orif they include equivalent structural elements with insubstantialdifferences from the literal languages of the claims. As various changescould be made in the above compositions and methods without departingfrom the scope of the disclosure, it is intended that all mattercontained in the above description and shown in the accompanying figuresshall be interpreted as illustrative and not in a limiting sense.

When introducing elements of the present disclosure or the preferredembodiments(s) thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

What is claimed is:
 1. An adhesive composition suitable for use as athermal conductor for electronic components, the composition comprisinga first plurality of carbon-based particles and a second plurality ofcarbon-based particles, the first plurality of carbon-based particleshaving an electrically-insulating coating having a thickness of at leastabout 10 nanometers extending over the carbon-based particles of thefirst plurality such that a continuous coating ofelectrically-insulating material is formed, and the second plurality ofcarbon-based particles uncoated with the electrically-insulatingcoating, wherein a ratio of the first and second pluralities ofcarbon-based particles in the adhesive composition is selected to ensurean electrical conductivity of the adhesive composition is less than apredetermined threshold, the first plurality of carbon-based particlesof the adhesive composition having an electrical resistance of at least50 ohms*cm and comprising at least about 10 percent of the adhesivecomposition by weight.
 2. An adhesive composition as set forth in claim1 wherein the carbon-based particles are in a shape selected fromplatelets, fibers, spheres, flakes, tubes, rods, diamonds andcombinations thereof.
 3. An adhesive composition as set forth in claim 1wherein the carbon-based particles comprise carbon fiber.
 4. An adhesivecomposition as set forth in claim 1 wherein the carbon-based particlescomprise graphitic carbon.
 5. An adhesive composition as set forth inclaim 4 wherein the graphitic carbon is a material selected from carbonfibers, graphite, exfoliated graphite, graphene and combinationsthereof.
 6. An adhesive composition as set forth in claim 1 wherein theadhesive composition is a paste.
 7. An adhesive composition as set forthin claim 1 wherein the electrically-insulating coating is selected fromceramics and diamond-like carbon.
 8. An adhesive composition as setforth in claim 1 wherein the electrically-insulating coating is aceramic selected from the group consisting of metal oxides, metalnitrides, metal carbides and combinations thereof.
 9. An adhesivecomposition as set forth in claim 1 wherein the electrically-insulatingcoating is a ceramic selected from boron carbide, boron nitride, siliconcarbide and silicon nitride.
 10. An adhesive composition as set forth inclaim 1 wherein the carbon-based particles are entirely coated with theelectrically-insulating coating.
 11. An adhesive composition as setforth in claim 1 further comprising a binder.
 12. An adhesivecomposition as set forth in claim 11 wherein the binder is selected fromepoxies, polyurethanes, acrylics and combinations thereof.